I can only speak about Bar Associations in general and the Washington State Bar Association in particular. The Wiki article is accurate in that the State Supreme Court decides the minimum criteria that must be met by any person wishing to practice law in that State. The Bar Association enforces those criteria, administers the Bar Examination, collects mandatory membership dues, keeps track of mandatory Continuing Legal Education credits, monitors lawyer general and IOLTA accounts, provides help and support to all members, and prosecutes lawyers who violate the rules of professional conduct. It is governed by a Board of Governors, and it has numerous Standing Committees made up of volunteer lawyers.

Although membership is mandatory, the Association does not dictate to law firms which lawyer(s) it should hire and how much to pay them. In that regard, it's every man for himself. Some choose to practice law by themselves, while others decide to join a firm. Still others collaborate either formally or simply share offices. In my mind, being a professional means that nobody but yourself determines how much you are paid. If you are good and can get those hard cases, you should be properly remunerated.

The idea proposed by the OP would work to some extent. It would have to mandate that ALL pilots be members, which is a huge hurdle not easily overcome. How would you deal with the fact that there are pilots all over the country? It's difficult enough to manage all lawyers in one single state. Crossing state lines to do that would be a bureaucratic nightmare. And what about military pilots who are transitioning to airlines? Would the guild oversee flight schools to ensure that standards are met?